Low-mass stars and extra-solar planets

The study of Exoplanets is one of the most dynamic and exciting areas of modern astronomy. Since the first discovery in 1995 of a planet around a normal, sun-like, star, more than 800 planets have been discovered. So far most planets have been discovered through the reflex motion they induce in their host star (as they orbit their common centre of mass), and while this technique gives orbital periods, without the orbital inclination almost nothing is learned about the planet itself.

Astronomers within ARC use (and operate) the SuperWASP instruments to look for tiny dips in the brightness of stars due the passage of an orbiting planet and obscuring a fraction of the stellar light (a so-called transit) and this automatically tells us, with excellent accuracy, the inclination. Hence the equations of orbital motion can be solved and the mass and radius and hence density of the planet determined (and compared directly with theoretical models of planet composition).

Developments within the SuperWASP project mean that our instruments will be sensitive to planets from roughly 20 Earth mass and larger. The smallest planets will probably remain only detectable from space. Currently two missions CoRoT and Kepler and flying and finding new planets (see picture above - an artists impression of the smallest CoRoT planet so far). ESA also have plans for a planet detection mission: PLATO is designed to find terrestrial planets in the habitable zones of bright (and nearby) solar like stars. ARC astronomers are at the heart of this mission as its goes through its development phases.

Ever a cursory glance at the solar surface shows it to be a turbulent place with granules, flares, spots, pulsations etc. While these phenomena are interesting in their own right telling us about the nature of own own star, they also add “noise” or “jitter” to spectroscopic observations making the determination of small variations in reflex motion difficult. Other stars suffer from these phenomena to a greater (or sometimes lesser) extent. None the less, in the future the detection of the smallest planets will be critically dependent on an understanding of the effects of these phenomena and hence how to filter them or minimize their effect on observations of the reflex motion. ARC astronomers are exploring new ways to understand these effects and hence minimize them.

The eclipses that transiting planets undergo can also be used for further characterisation. For example, observations of the transit at different wavelengths can tell you something about the radius of the planet at that wavelength, which can be used to constrain the planetary atmospheric structure. Observations during the anti-transit (planet passing behind the star) can be used to derive the temperature of the planet directly while changes in the brightness of the planet during its orbit tell us how reflective the planet is. ARC astronomers are exploring these effects and trying to probe the atmospheric constitution of extrasolar planets.